1. Field of the Invention
This invention relates to colloidal siliceous composition and to its use in a process for the production of paper and paperboard.
2. Brief Description of the Prior Art
Conventional paper or paperboard manufacture involves forming a fibrous stock containing additives such as pigments, fillers and sizing agents and dewatering the stock on a metal or fabric wire to form the basis for the paper or board sheet. Such processes have been subject to the conflicting requirements that ready drainage of the stock should occur and that there should not be undue loss of additives and of fibre from the stock in the course of drainage, that is, that the retention of such additives and fibre on the wire should be high. This acts not only to give a saving in raw material costs and a reduction in the energy required to dry the sheet but also reduces effluent treatment requirements as a result of a lower content of suspended solids, and lower COD and BOD loadings, in the purge water. Sheet formation and surface properties may also be improved. There have been many attempts to optimise drainage and retention properties by the use of combinations of additives, which include polyelectrolytes such as high molecular weight polyacrylamide and its copolymers, which act as flocculating agents.
It has been proposed to use colloidal swelling clays in conjunction with the high molecular weight, relatively low charge density polyacrylamides which have traditionally been used as flocculants, which may be nonionic, anionic or cationic in nature and may be selected to suit the charge demand of the stock.
U.S. Pat. No. 3052595, for example, discloses the addition of bentonite to filled stock followed by an acrylamide homopolymer or copolymer which may include at most about 15% by weight of a functional comonomer which may be anionic or cationic in nature, corresponding to a charge density of at most about 2 m.eq./g. The affect of the above combination is that the polymer and the bentonite "are mutually activating whereby increased retention of the filler in the paper web and decreased turbidity of the resulting white water are obtained".
More recently, European Patent Specification No. 0017353 disclosed that the fibre retention and dewatering properties of substantially filler-free stocks may be improved dramatically by including in the stock a high molecular weight; e.g. a molecular weight essentially above 100,000, normally above 500,000 and generally about or above 1 million; polyacrylamide and a bentonite-type clay. The polyacrylamide may contain not more than 10% of either cationic or anionic units and is limited thereby to low charge density material.
This line of development has hitherto culminated in the process described in European Patent Specification No. 0235893 comprising adding a high molecular weight linear cationic polymer to thin stock in a quantity which is greater than that conventionally used to form large flocs, subjecting the flocculated suspension to significant shear and adding bentonite to the sheared suspension. It is explained that the effect of shearing is to break the flocs down into microflocs which are sufficiently stable to resist further degradation.